In recent years, many Turkish universities, particularly the foundation universities in the private sector, have adopted English either in part or in full, as the language of instruction. In practice, this has meant that English should be the only working language of instruction in all academic activities, ranging from lectures, seminars, presentations to thesis defenses. However, little attention has been paid to the fact that the ideals of policymakers are not always in tune with the actuals of the policy implementers ( Jenkins, 2014; Karakas, 2016a). In the Turkish higher education, there is evidence that lecturers and students often breach the English-only policy by using a mixed-version of Turkish and English, which is widely known as Tarzanca (Tarzanish in English) in Turkey (Collins, 2010; Karakas, 2016b). This critical review seeks to explore the phenomenon of Tarzanish in general and its use in English-medium instruction (EMI) universities in particular. While doing so, it is also aimed to find answers to the following questions: (1) How is Tarzanish conceptualized and described in the dictionaries, literature, online sources (e. g. blogs, discussion forums, etc.) and by scholars? (2) What are its descriptive characteristics in terms of morphology, syntax, and lexis? (3) Why do EMI people (lecturers and students) resort to it? Moreover, (4) what can be done to resolve the issue of Tarzanish in EMI universities? Drawing on the answers to these questions, the paper suggests that the notion of Tarzanish means different things to different people, lay people and EMI people resort to it for different purposes, and its use by lay people and EMI people show divergences due to some variables such as the level of language proficiency and the domains of language use (e.g., tourism, business, and higher education). Finally, some suggestions have been offered for the solution of the issue of Tarzanish in EMI universities.